The 15x7 inch snowflake rims came in two different variations. Both were 15x7 inches in size, but there was a second design released in 1979 that was a thinner/lighter snowflake rim. Most people do not know the difference, but when you put them side by side, you can definitely see the difference. Here you can see the difference between the 15x7 inch snowflakes for 1979-81 (left) and 1977-81 (right). You can tell from far away by the inner most circle has edges on the right and not the left. Also, the outer most circle has openings on the left, but not on the right. Now as far as which rims were used on which cars and why they still had both styles in 1979 to 1981 is still left to be determined.

So, if you are looking to purchase a set of rims off of someone from the Internet, make sure you look closely at the photos to make sure the rims you are purchasing are all the same "style". I have seen plenty of auction on Ebay or ads for rims for sale where there is a mismatch of rims for sale. So study the photos closely and you will be able to tell if all fours trims that you are purchasing are the same.
In 1978, Pontiac came out with the WS6 handling package which included new 15x8 inch snowflake rims. They do look similar in nature, but are definitely different sizes as you can see in the photo below. The 15x8 inch snowflakes (shown on the right) had a 1 inch lip surrounding the outside edge of the rim that gave it the extra 1 inch depth.
Now again, just like in 1977, these rims could only be ordered in the agrent (grey) or gold colors. The gold rim was typically used on the Special Edition Trans Ams, but you could order gold 15x8 inch snowflake rims and any Trans Am from 1978 to 1981 with the WS6 handling package.

In 1978 when the gold rims first came out, they were painted all gold except for the outer lip of the rims. Typically you see this style of gold rim on the Y88 Gold Special Edition rim, but this was also true for any gold rim ordered in early 1978. As you can see from the photo below, the rim on the left was the all gold rim used in early 1978 that was painted except for the outer lip, and the gold rim on the right used late 1978 through 1981 is painted gold only in the recessed areas of the rim.
Like was said above, these gold rims were typically seen on the Y88 Gold Special Edition Trans Ams, but they could also be ordered on any Trans Am with the WS6 handling package. Now when they made the change from the all painted gold rims to the machined edged gold rims is anyone's guess right now. My research has shown that it happened sometime in 1978, but when and why they made the change has yet to be seen. We do know in 1979 the all painted gold rims were gone and they were all the machined edge for the gold rims. The argent (grey) rims never had this type of a paint scheme or change, it was only the gold rims.

There are also differences in the 15x8 inch rims as well. As you can see in the photos below, the edges are different on different rims. The 15x8 rims from early 1978 had sharp corners around the edges and the ones from late 1978 and later had more of a rounded edge. The spokes in the middle are also different. The early style had 1/4 inch thick spokes in the center, the later style had 3/8 inch spokes in the center. It isn't by much, but you can definitely see a difference. Also, there is more even in the later style rims. You can't really notice because they look almost exactly alike but some I have seen are 5/16 inch thick spokes, and the others are 3/8 inch spokes. I have found at least 4 different variations on the 15x8 snowflake rim (maybe 5). So, if you are looking to buy or replace just one or two wheels, make sure you get the right ones.

Sharp corner edge and 1/4 inch spokes	Rounded corner edge and 3/8 inch spokes
5/16 inch spokes	3/8 inch spokes

Some people point out that you can also tell the difference by the location of the lug hole in relation to the valve stem. If you notice in the photos below, the rims have different locations of the lugnut holes. Some are to the right of the valve stem, and some are to the left of the valve stem.

There are slight differences between the 15x7 Turbo Rims and the WS6 15x8 Turbo Rims. It is hardly noticeable but when you line the two up together, you can definitely see the difference. From the top looking down at the Turbo rims, it is hard to tell. In the photo below you can't even really see a difference can you? Ok, besides one is real shinny and the other one is real dull.

The difference between the rims really comes in the depth of the front dish. The rims are real close in height. They are both 16 inches across, but the 15x7 Turbo rim is only 8 1/2 inches tall whereas the 15x8 Turbo rim is 9 inches tall.
The difference come in the front dish depth of the rim. They both have a 4 1/2 inch back spacing to them, but the front spacing is different. The 15x7 Turbo only has about a 3 1/4 inch depth and the 15x8 Turbo has about a 3 3/4 inch depth. It is really noticeable when you look closely at them.
So, if some one is selling you a set of Turbo rims and you need the 15x8 inch ones for 10th Anniversary cars, then ask them the EXACT measurements on the height of the rim. Some people measure them and see that they are 8 1/2 inches tall, so they figure they have 15x8 inch Turbo Rims... Well, they aren't. Don't take round numbers like "Well, its about 9 inches...", ask them the EXACT height.

The last thing to take a look at is the measurements for the rims. I get alot of questions on people who want to change rims and ask what other type of rim will fit. Well, it would be too long of a list to list every type/style/manufacturer of rim that would fit the Trans Ams and Firebirds. So what I will cover is how to measure the rims to know if the rim will even fit the car. The first thing to take a look at is the lug pattern. The GM cars like the Firebird and Trans Am used the 5 on 4 3/4" lug pattern. What does this mean? It means there were 5 lugs that held the rim on and they were spaced at 4 and 3/4 inches apart. So looking at the rim, you can easily tell if it has 5 lugs or not. But the correct spacing for measuring the lug pattern is done by measuring from the back on one hole to the center of the hole diagonal to it (shown below). So, on all of the snowflakes, honeycombs, turbo, and rally rims, the measurement was a 5 lug pattern spaced at 4 3/4 inches apart.

The next thing to take a look at is the offset and backspacing for the rims. The backspacing is easy to measure. Place one of your rims up on its edge and take a measuring tape and place the end of it flush to the back inside of the rim where the lug nuts would attach (where it would sit flush against the hub or drum of the car). Then take a look straight down the edge of the rim and see where the edge of the rims line up and take your measurement (shown below). As you see in the photos below, this rim has a 4 3/4 inch backspacing.

The last item to take a look at when thinking about changing rims on your car is the offset of the rim. The off set whether it be positive or negative does make a difference on how the car handles. To determine the offset of the rims, you first need to measure the width of the rim from edge to edge. Divide that in half to get the centerline of the rim. Now take your backspacing and subtract the measurement of the centerline of the rim from it to get your offset. So, in the example shown below of a 15x8 snowflake rim, you see that the rim is 9" from edge to edge which means the centerline is 4 1/2". Take the backspacing of 4 1/2" and subtract the centerline measurement of 4 1/2" and it gives you an offset of + 0". So, the simple math is: offset = backspacing - (width of rim /2).

So, once you know the lug pattern, the backspacing, and the offset of the rims, then you can determine what rim you would be able to fit on your car. The measurements of the stock rims on the Firebirds and Trans Ams are as follows: